The present invention relates to a hydraulic pressure control system for controlling shocks occurring upon engagement of a torque transmitting frictional element that is engaged to effect shifting between gear positions in an automatic transmission.
Vehicular automatic transmissions are known wherein in response to the vehicle operating conditions shifting between gear positions is effected automatically by supplying hydraulic fluid pressure to one or more selected frictional elements. U.S. Pat. No. 4,502,354 (corresponding to JP A-No. 57 47056) discloses a hydraulic pressure control system wherein in order to reduce shift shock during a change in gear position, the hydraulic pressure (a line pressure) for actuating a frictional element is maintained equal to the magnitude of a target hydraulic pressure for a predetermined holding time.
According to this known control system, there is no fine control of the hydraulic fluid pressure during a period when the hydraulic fluid pressure returns to the level of the line pressure so that there occurs a change in transmission output torque during this period and this change is felt as a shift shock.
An object of the present invention is to provide a hydraulic pressure control system which can alleviate a change in transmission output torque when the hydraulic fluid pressure returns to the line pressure.